This study attempts to define element fractionation when analysing tephra glass shards using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). Sixteen MPI-DING and USGS reference materials and natural tephra samples have been repeatedly analysed using a 193 nm Excimer laser coupled to a magnetic sector ICP-MS. 29Si is a suitable internal standard for the analysis of rhyolitic glass shards using <10 μm crater diameters, whilst 43Ca and 44Ca are only suitable internal standards for the analysis of basaltic glass using >10 μm crater diameters.
Element fractionation becomes significant at <20 μm crater diameters and generally increases with decreasing crater diameter. Relative to certified concentrations; Zr is typically underrepresented by ~10% at 10 μm crater diameters, ~20% at 6 μm crater diameters and ~25% using 4 μm crater diameters. Elements fractionate differently; yet there is no correlation between fractionation and ionic radius, field strength, condensation temperature, electronegativity or ionization energy. Higher excitation voltages increase the fractionation of 20 μm and 10 μm crater diameters, possibly associated with a concomitant increase in laser pulse length. However, excitation voltage does not influence the fractionation of <10 μm crater diameters. Polymerisation, viscosity, liquidus temperature and iron concentration do not exert a systematic control on element fractionation.
The systematic influence of crater diameter on fractionation allows for its correction. Fractionation factors have been calculated from the repeated analysis of 16 samples and have been applied to Minoan (rhyolite) and UT2252 (basalt) glass shard analyses. Application of these fractionation factors brings concentrations measured from 6 μm crater diameters to within ±10% of concentrations from 20 μm crater diameters. Precisions from 20 μm and 10 μm crater diameters are good, at ±10-20%, whilst analyses performed at < 10 μm crater diameters are subject to ±20-50% errors.
Fractionation factors calculated here are also applied to the 10 μm and 6 μm crater diameter trace element analyses of Ulleungdo (Korea) proximal pyroclastic Units 2, 3 and 4. Ulleungdo units share similar trace element chemistry, thus further glass and mineral analyses are required in order to correlate further with the Suigetsu SGO6 (Japan) core.
|Date of Award
|Nick Pearce (Supervisor)